This invention relates to a resistive circuit element made according to the so-called "thick-film" technology by superposition of two films, one conductive, the other resistive, to obtain a resistance that withstands strong voltages and that is brought to a precise ohmic value by several laser cuts correctly distributed in the resistive film.
"Thick-film" hybrid technology has as its principal base materials resistive, conductive and insulating inks. These inks are in the form of pastes that contain the following elements: special powdered glass, pulverulent precious metals, organic binder, diluent consisting of a mixture of solvents. These ingredients which are mixed to form a thick paste are deposited on ceramic plates called substrates, generally of alumina, by the process of silk-screen printing. Once the paste is deposited on the substrate, the piece is dried at 100.degree.-150.degree. C. to remove the solvents from it and fired in a furnace at 500.degree.-1,000.degree. C., generally 850.degree. C. During the firing, three phenomena occur: breakdown of the organic binder, sintering of the glass particles on the surface of the substrate and vitrification of the unit. Thus, the elements that make up the circuit adhere very strongly to the ceramic.
A resistor made in "thick-film" hybrid technology is shown in FIG. 1 of the accompanying drawings. It comprises two different films deposited on a substrate 1: the first 2a, 2b, made of a silk-screened conductive ink, dried and optionally fired, serves as a support and as terminals for the resistor; the second 3, made of a silk-screened resistive ink, dried and fired, is in itself the actual resistor. These two films, if the method of manufacturing allows it, can be co-fired, i.e., fired together.
This technology makes it possible to make resistors in a very wide value range (10-10.sup.6 .OMEGA.) depending on the choice of the type of resistive ink used and on the variation of the geometry of the printed resistors.
The materials going into the composition of the conductive ink for the conductive film have a base of metals or alloys such as silver, palladium, platinum, gold, copper, aluminum. The choice of these various metals rests on several criteria: solderability, resistance to aging, definition for printing, low resistivity, adherence to the substrate, compatibility with the resistive ink used and possibility of annealing. The thickness of the conductive film is generally between 5 .mu.m and 50 .mu.m.
The most used materials going into the composition of the ink for the resistive film are metal oxides such as ruthenium oxide or pyrochlores such as thallium ruthenate, whose principal parameters are resistivity, heat variation coefficient, stability over time. The thickness of the resistive film is generally between 10 and 30 .mu.m.
This "thick-film" hybrid resistor can be adjusted by means of a medium-power (0-5 watts) laser beam. This technology of laser cutting consists in vaporizing the resistive materials by creating high intensity coherent light pulses of short duration. A series of laser pulses that more or less overlap creates a narrow groove (on the order of 50 .mu.m) that goes through the resistive film to the substrate and thus cuts the resistor. This cut deflects the lines of current that go through the structure, thereby increasing its ohmic value, and the totality of the voltage applied to the resistor is found on both sides of the laser groove.
The two major problems encountered with this type of cutting for high-voltage resistors are therefore the creation of one or more hot spots accompanied by microcracks at the top of the cutting or cuttings where the concentration of the lines of current are located, and the appearance of an electric arc while operating, from one edge to the other of certain laser cuts when the electric field exceeds a certain limit (on the order of 3,000 v/mm in dry air).
FIGS. 2A and 2E of the accompanying drawings show several forms of cuts which were the object of experimental tests on small-sized "thick-film" hybrid resistors subjected to voltages of several hundred volts. These forms of cuts have proven unsuitable because there resulted either the creation of hot spots at 4a, 4c, 4d, 4g, 4h, 4i, 4j, or too strong a voltage gradient between the two edges of the laser groove marked 4b, 4e, 4f, 4k, 4l, that could cause a poor stability or the destruction of the resistor, by appearance of an electric arc.
With these forms of cuts, said problems can be solved only by oversizing the resistor, which is not always compatible with the installation capabilities offered and increases the manufacturing costs.